The invention is related to anchors and more particularly to gravity anchors that are used to moor barges and vessels in place.
Floating marine vessels and barges are subjected to external loads from the environment and from operation of the vessels. Environmental loads are mainly from waves, wind, and current. Operation loads originate from the function, such as towing, laying of submarine pipelines, etc. It is essential to hold the vessel at the desired designated station (station keeping) in order to execute the work offshore. “Station keeping” of the vessel is achieved by two methods—mooring by marine anchors and by dynamic positioning means such as thrusters.
Mooring by marine anchors is typically the simplest way of station keeping a vessel. Various types of anchors have been used for this purpose. One type is the “gravity anchor” or the “deadweight anchor”.
Anchor handling tug boats (AHT's) are the typical vessels/boats used to deploy and retrieve anchors.
The gravity anchor provides holding power from the submerged weight of the anchor.
The holding power of the gravity anchor is proportional to its weight. The higher the weight, the higher the holding power of the anchor. However, the higher the weight of the gravity anchors, the more difficult it becomes to handle it offshore. Often, the tugs (AHT's) may not have the required crane capacity to handle these heavy gravity anchors. Due to the size of the vessels involved in offshore oilfield work, gravity anchors can each easily weigh several tons in order to provide the necessary holding power.
The disadvantage of the need for AHT's is compounded in shallow water operations where the draft of the AHT must be limited. This tends to contradict the crane capacity required on the AHT to effectively handle the anchors.
Because conventional gravity anchors must be lifted by crane, re-deployment becomes a time consuming operation. Deployment of gravity anchors by and storage on the vessel presents issues related to storage space due to the size and placement of multiple anchors and the potential damage that the anchors can cause to the vessel during deployment and retrieval of the anchors.